Development and characterisation of a state-of-the-art GOME-2 formaldehyde air-mass factor algorithm

[摘要] Space-borne observations of formaldehyde (HCHO) are frequently used toderive surface emissions of isoprene, an important biogenic volatileorganic compound. The conversion of retrieved HCHO slant columnconcentrations from satellite line-of-sight measurements to verticalcolumns is determined through application of an air mass factor (AMF),accounting for instrument viewing geometry, radiative transfer, andvertical profile of the absorber in the atmosphere. This step in thetrace gas retrieval is subject to large errors. This work presents theAMF algorithm in use at the University of Leicester (UoL), whichintroduces scene-specific variables into a per-observation fullradiative transfer AMF calculation, including increasing spatialresolution of key environmental parameter databases, input variablearea weighting, instrument-specific scattering weight calculation, andinclusion of an ozone vertical profile climatology. Application ofthese updates to HCHO slant columns from the GOME-2 instrument isshown to typically adjust the AMF by ±20 %, compared to areference algorithm without these advanced parameterisations.On average the GOME-2 AMFs increase by4 %, with over 70 % of locations having an AMF of 0–20 % larger than originally,largely resulting from the use of the latest GOME-2 reflectance product.Furthermore, the new UoL algorithm also incorporates a full radiativetransfer error calculation for each scene to help characterise AMFuncertainties. Global median AMF errors are typically 50–60 %, andare driven by uncertainties in the HCHO profile shape and its vertical distribution relative to clouds and aerosols.If uncertainty on the a priori HCHO profile is relatively small (< 10 %) thenthe median AMF total error decreases to about 30–40 %.

[学科分类] 几何与拓扑